Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD). Remarkably, the lifetime risk estimate of developing AD for individuals with two copies of the apoE4 allele is ~70% by the age of 85. By comparison, the lifetime AD risk estimate for individuals with two copies of the apoE3 allele is ~10% by the age of 85. Although many apoE4 homozygotes do go on to develop early-onset AD (EOAD, <65 years) or late-onset AD (LOAD, >65 years), some of them (~20%) stay asymptomatic over age 85. Understanding the susceptibility of the former apoE4 group and the resistance of the latter apoE4 group to AD might allow for the development of strategies to prevent or delay the disease in people at risk for AD. One of the pathological hallmarks of AD is the formation of neurofibrillary tangles (tauopathy). A major difficulty of studying AD-related tauopathy is that overexpression of wild-type (WT) tau in mice or cultured cells does not lead to tauopathy. Thus, almost all published studies used mice or cells overexpressing tau with different mutations to model tauopathy in AD. However, all those tau mutations cause frontotemporal dementia (FTD) but not AD, raising concerns on their mechanistic accuracy. Until recently, tau-A152T polymorphism was identified, which is associated with increased risk for both FTD and AD. Thus, studying tau-A152T-related pathophysiology should shed light on the pathogenesis of tauopathy in both AD and FTD. While animal models and post-mortem tissues have provided key insights into the pathogenesis of AD, they also pose strategic limitations. Induced pluripotent stem cells (iPSCs) derived from human somatic cells with AD-linked mutations or polymorphisms, together with genome-editing techniques, hold great promise as in vitro models for studying disease pathogenesis in relevant cell types, including human neurons. This proposal aims to capitalize on this promise by building on our preliminary studies showing that: (1) AD patient iPSC- derived neurons with an apoE4/4 genotype developed AD-related phenotypes in culture, including increased A levels and elevated tau phosphorylation, compared to apoE3/3-iPSC-derived neurons; and (2) iPSC- derived neurons with tau-A152T developed tauopathy due to increased tau fragmentation. Specifically, we will determine the phenotypic differences among human neurons derived from apoE4/4- iPSC lines from EOAD patients, LOAD patients, and asymptomatic controls <65 and >85 years of age (Aim 1); explore the transcriptomic and proteomic differences among human neurons derived from the isogenic iPSC lines with an apoE3/3 or apoE4/4 genotype (Aim 2); and determine the contribution of tau fragmentation to the pathogenesis of tauopathy in AD versus FTD (Aim 3). These studies, using human neurons and astrocytes derived from isogenic iPSC lines, will likely identify genetic factors capable of modifying apoE4's detrimental effects in AD pathogenesis and may discover new therapeutic targets for apoE4-associated AD. These studies should also provide insight into the pathogenic differences of tauopathies in AD versus FTD.